1. Field of the Invention
This invention relates to a hot rolling mill through which a heated piece of metal, such as steel, is passed to produce a progressively thinned and elongated metal strip which is then cooled, and more particularly, to an apparatus and method for cooling the strip to provide a uniform temperature distribution across the width of the strip.
2. Background of the Prior Art
It is the conventional practice to heat and then cool a hot rolled metal strip, such as steel, during the rolling process for the purpose of controlling the hot rolling process and improving strength, toughness and other properties of the hot rolled steel strip. One step in the rolling process is the controlled cooling of the steel strip which is typically done using a laminar flow of coolant, such as water, dispersed upon the top and bottom sides of the strip. The edges of the strip tend to cool first and to a greater extent, thereby providing a nonuniform temperature distribution across the width of the strip. While a small difference in the temperature between the edge and the center of the strip may be tolerable, large temperature differences are not, because not only does this provide non-uniform mechanical properties to the strip but furthermore results in shape defects such as waviness from what otherwise should be a flat steel strip.
As used herein, the term "strip" is used to identify steel in coil form or plates which are being rolled on a plate mill either from discrete slabs or as a coil plate product.
FIG. 1 is prior art and illustrates a portion of a hot rolling mill 10 with a section view of a steel strip 15 horizontally supported by a roller 20 which is itself supported by a roller shaft 25 mounted to roller supports 30. The steel strip 15 travels along a series of rollers 20 forming a roller table in a direction of travel out of the page as indicated by arrow 35. A supply pipe 50 has branching from it a number of coolant pipes 55, each extending over the steel strip 15 in a direction generally perpendicular to the direction of travel 35 of the steel strip 15. Extending radially from the coolant pipe 55 and distributed along the length of the coolant pipe 55 is a plurality of nozzles 60 directed toward the steel strip 15 for distributing coolant across the width of the steel strip 15. The coolant generally drains from the center over the outer edges 75 of the steel strip 15 and therefore the quantity of water at the center 70 of the steel strip 15 is less than the quantity of water at the edges 75 of the steel strip 15. As a result, the increased quantity of coolant at the edges 75 has a greater capacity to absorb heat at the edges 75 than the lesser quantity of coolant at the center 70 of the steel strip 15. This in itself may promote a non-uniform cooling across the width of the steel strip 15. Even without the non-uniform coolant flow over the steel strip 15, the edges 75 of the steel strip 15 would still cool faster than the remainder of the strip 15 because the center 70 is warmed by the adjacent portions of the strip 15 while an edge 75 receives such warming only on the side of the edge 75 toward the center 70.
The temperature of the steel strip as it exits the hot rolling mill is about 1500-1700.degree. F. After cooling, a temperature difference from the center of the strip to the edge of approximately 30.degree. F. is acceptable and is considered to represent a uniform temperature distribution. However, temperatures greater than that difference tend to excessively modify the metallurgical properties of the steel and also tend to promote waviness of the edges 75 of the strip 15.
Current cooling techniques involve utilizing a plurality of nozzles 60 across the length of the coolant pipe 55, each with the same inner diameter. This provides an acceptable temperature distribution for steel strip 15 having a width of 80 inches or less. FIG. 2 shows the temperature profile for an 80-inch section of the steel strip 15 indicated by letters B and C in FIG. 1.
The greatest temperature differences between the center 70 and the edges 75 occur very close to the edge 75. For typical plate product in which the strip is greater than 80 inches wide, such as, for example, 120 inches wide plate as illustrated by points A and D in FIGS. 1 and 2, the temperature difference in the edge region of the steel strip 15 is drastically different. This difference is unacceptable, and an apparatus and method to provide a more uniform cooling rate across the width of the steel strip 15 is desired.
FIG. 3 illustrates details of Section III--III shown in FIG. 1 wherein the supply pipe 50 provides to the coolant pipe 55 coolant which is disseminated through nozzles 60. Each of the nozzles 60 is equally spaced and furthermore all of the nozzles have the same internal diameter.
An object of this invention is to provide a method and apparatus which may be utilized to provide a uniform temperature distribution across the width of a steel strip during cooling upon exiting from a hot strip mill.